The interaction of polymeric materials such as urethane with the body has long been of interest to scientists, engineers, and clinicians. The success of a biomedical device depends upon its ability to withstand the physical and chemical environment to which they are subjected. Materials from which biomedical devices are made must exhibit sufficient biocompatibility to preclude rejection by the body or cause any harmful reaction such as inflammation or thrombus formation. The ability for a polymeric material to meet such criteria is dictated mainly by surface chemistry and to a lesser degree that of the bulk material.
Polyurethane elastomers are an example of a particular class of polymeric material that has found wide acceptance in biomedical applications. It has been established that the types of proteins adsorbed, and the nature of such adsorption, largely dictates the biocompatibility of the polyurethane material, assuming that other criteria related to biocompatability are met. Factors which affect the protein adsorption characteristics of polyurethane substrate include; processing, environment and urethane chemistry.
It has also been found that the method of fabrication of the material can alter its biocompatibility. For example, whether the material is cast or extruded can affect the blood contacting surface of the material and its thrombogenic properties. Extruded materials in certain instances are more thromboresistant than cast materials.
The thrombogenic properties of materials has commonly been related to the nature of proteins the surface absorbs. It has been demonstrated that certain classes of protein tend to decrease the thrombogenic properties of a material when adsorbed. For example, surfaces which preferentially adsorb albumin have been found to be relatively thromboresistant, while those surfaces which absorb fibrinogen tend to be highly thrombogenic. In the prior art, attempts have been to made to alter the protein adsorption characteristics of the materials so as to decrease the thrombogenic properties. Various prior art surface treatments include; chemically grafting certain functional chemical groups to the polymer surface, hydrogel surfaces, attachment of pharmacologically active antithrombogenic surfaces and physical modifications. Such methods of surface treatment modifications have been used as a means of grafting or fixing a thin surface film. Such surfaces have typically been subjected to high energy treatment such as ion implantation or plasma deposition. These high energy treatments are used to treat the surface such that the surface becomes reactive and bonds with the film to be attached. The benefit of such films is that they are ultra-thin, are strongly bonded to the substrate and usually do not alter the mechanical properties of the base material. While such surface treatments as plasma or ion deposition have been used to affix a layer as described above, the prior art has not taught the use of these energetic treatments to alter the protein adsorption characteristics of the material.
It has also long been desirable in the orthodontic industry to improve the aesthetic properties of elastomeric orthodontic devices. Recently, it has become apparent that a great demand exists for orthodontic devices which have a minimal visual impact when viewed by others. As a result, a new generation of orthodontic brackets which are difficult to visually observe, have become much more available. Typically, such orthodontic brackets made out of ceramic or crystalline materials having a substantially clear appearance or a color substantially identical to the tooth. While such orthodontic devices have contributed significantly to improving the aesthetic appearance of orthodontic brackets, the elastomeric orthodontic tensioning devices used therewith, such as elastomeric O-rings and chains, detract or diminish the improvement such devices have provided due to staining and/or discoloration. Clinical experience has indicated that such ligatures and chains take on a discoloration or stained appearance within one to two weeks after application of the device. It is believed that the discoloration of these devices occur due to the same phenomenon that allows plaque formation. It is well known that the first step in plaque formation is the deposition of a protein layer to which larger macro molecules and/or other proteins adhere to. Their adhesion and subsequent denaturalization leads to a stained or discolored appearance typically associated with plaque. Therefore, it is desired to vary protein absorption to avoid this problem. With more aesthetic orthodontic brackets, discolored or stained ligatures become more noticeable, and thus detract from the overall appearance and benefits provided by the orthodontic bracket. While it is known that certain materials provide staining qualities, these materials lack the necessary strength to be used as orthodontic ligatures. Applicants have found that the stain resistant qualities of the orthodontic ligature made of an elastomeric urethane can be improved by reducing the protein adsorption of the ligature.
Applicants have invented an improved dental and biomedical devices wherein the protein adsorption characteristics thereof can be altered so as to provide improved biomedical compatibility or improved stain resistance without detracting from the strength of the material or product.